Electrographic printing system with plural staggered electrode rows

ABSTRACT

In an electrographic printing system, a multiple row electrode structure wherein successive rows are mutually spaced from each other, each row including mutually spaced electrodes, the electrodes of successive rows being positioned in a staggered manner with respect to each other. The system further comprises improved electrode drive circuitry, including a plurality of high-voltage drivers and a selection matrix wherein a plurality of passive elements are coupled to the drivers. A plurality of output lines couple the matrix to the electrodes so as to selectively apply a high voltage to the electrodes in order to produce a latent image on a dielectric medium. Toning means subsequently make the latent image visible.

United States Patent [72] Inventors Michael S. Shebanow;

Ronald F. Borelli, both of Mediield, Mass. [2!] Appl. No. 824,4l9 [22]Filed May 14, 1969 [45] Patented Nov. 30, 197i [73] Assignee HoneywellInc.

Minneapolis, Minn.

[54] ELECTROGRAPIIIC PRINTING SYSTEM WITH PLURAL STAGGERED ELECTRODEROWS 17 Claims, 12 Drawing Figs.

[52] U.S. Cl 346/74 ES,

l0 i/DIG. l3 5| Int.Cl ....G03g 15/00 [50] Field of Search 346/74 ES[56] References Cited UNITED STATES PATENTS 2,934,673 4/1960 MacGritf346/74 3,157,456 llll964 Kikuchi 346/74 ABSTRACT: In an electrographicprinting system, a multiple row electrode structure wherein successiverows are mutually spaced from each other, each row including mutuallyspaced electrodes, the electrodes of successive rows being positioned ina staggered manner with respect to each other. The system furthercomprises improved electrode drive circuitry, including a plurality ofhigh-voltage drivers and a selection matrix wherein a plurality ofpassive elements are coupled to the drivers. A plurality of output linescouple the matrix to the electrodes so as to selectively apply a highvoltage to the electrodes in order to produce a latent image on adielectric medium. Toning means subsequently make the latent imagevisible.

PATENTEBW 30ml 3,624.661 SHEET 10F 5 v I INvrzN'mRs Fig. l. RONALD F.BORELLI MICHAEL s. SHEBANOW AT ()RNIZY PAIENTEUNIN 30 I97] SHEET 2 OF 5v SCAN LINE 16 SCAN LINE 25 W m O 0L N LA \EB i M 5 mm A H O l R M 2 U lATTORNEY PATENTEU 30 3 624. 661

SHEET 30F 5 MICHAEL S. SHEBANOW PAIENTEDNIW 3 I9?! 3.624.661

SHEET nor 5 POSITION INPUTS 7O DATA INPUTS 72 \ELECTRODE STRUCTURE F\SEQUENTIAL OUTPUTS(192 x11) POSITION INPUTS\ 7 0 82b 85b 88b 81b 84b87b 1 83b 86b 89b r v 1 W q DATA INPUTS 82o 87 SEQUENTIAL 85o. 82OUTPUTS as INVI'INI'HRS RONALD F BORELLI MICHAEL S. SHEBANOWELECTROGRAPIIIC PRINTING SYSTEM WITH PLURAL STAGGERED ELECTRODE ROWS-BACKGROUND AND OBJECTS OF THE INVENTION The present invention pertainsgenerally to the field of image reproduction and relates moreparticularly to an electrographic printing system wherein the medium tobe recorded on includes a conductive base substrate and a dielectriclayer.

There presently exist various specific techniques of image reproductionmost of which are concerned with the electrostatic transfer of charges.Generally speaking in electrographic printing systems, including oneemploying the principles of the present invention, a latent image isformed on a dielectric medium by placing the medium in the fieldestablished between twoelectrodes. These two opposed electrodes whichcan assume various shapes (round, square, character-shaped, etc.) have ahigh electrical potential'difference applied across them, therebyestablishing the necessary field. The latent charged image fonned is inthe shape of the electrode that faces the dielectric surface of themedium.

Most electrographic printing systems can generally be categorized intotwo areas with respect to electrode configura tions; systems employingcharacter-shaped electrodes and systems employing pin-shaped electrodes.In systems of the former type, for example, a print drum is rotated athigh speed and selected electrodes are pulsed when the desired characteris facing the dielectric surface, causing the formation of a latentimage on the medium at the area where the character electrode waslocated. Associated with such systems, however, are certaindisadvantages. For instance, it has been found necessary to provide adrum consisting of individual, electrically insulated segments and toprovide means for commutating to each segment of the rotating characterdrum. Thus there are mechanical problems associated with the rotatingdrum. In addition, in order to operate at a reasonable printing speedwithout causing character smear, the duration of the selection pulsemust be short and the paper has to remain stationary during printing.This results in a printing speed limitation with a system of this type.

In printing systems where an array of pin electrodes is employed,various patterns, such as alphanumeric characters, can be reproduced byselecting predetennined electrodes as the recording medium passes theseelectrodes. Associated with such a system, however, are certain problemsthat result in a poor quality printout. Two of the more significantproblems derive from the poor contrast density and resolution of thefinally printed characters. The contrast (shade) density may be definedas the degree of darkness as represented on the gray scale, whileresolution may be defined as the capability of forming perfectly shapedcharacters. Theoretically, 100 percent contrast provides printedcharacters which are perfectly black. The pin electrodes have to beseparated a minimum distance from each other, so that they can beselectively activated. If this is not the case, poor density and poorresolution of the printed character result.

Another problem associated with a pin electrode system derives from thefabrication of the electrode array itself. Usually the electrodes arevery small in cross section and are located close together. Thestructure, therefore is prone to damage and is generally difficult tofabricate.

The cost of presently available electrographic printing systems isrelatively high. One of the important factors contributing to the highcost of such systems is the necessity for providing one driver perelectrode. In systems requiring relatively high resolution and shadedensity, the total number of electrodes may be of the order of 200electrodes per inch. Where a separate high-voltage driver is requiredfor each of these electrodes, the cost of the system becomesinordinately high.

It is an object of the present invention to provide an electrographicprinting system which is not subject to the foregoing disadvantages.

It is another object of the present invention to provide anelectrographic printing system which provides a printout of improvedquality with respect to resolution and shade density.

It is-a further object of the present invention to provide anelectrographic printing system which is capable of operating atrelatively high speeds.

It is still another object of the present invention to provide a novelelectrode structure for such a printing system which can be easily andaccurately fabricated at relatively low cost.

It is a further object of the present invention to provide an economicalelectrographic printing system which has improved reliability.

SUMMARY OF THE INVENTION The foregoing objects are satisfied in thepresent invention by providing an electrographic printing system of thekind wherein a recording medium has latent images formed thereon by theapplication of a high potential across the medium and wherein a toner issubsequently applied to the medium to make the latent images visible.The printing system further comprises a multiple row electrodestructure, a character generator, and a selection matrix which permitsthe use of a smaller number of high-speed drivers than the total numberof electrodes in the electrode structure.

The printing system which constitutes the subject matter of the presentinvention permits printing with improved resolution by providing theheretofore unattainable capability of printing between adjacentelectrode areas. A staggered electrode structure is employed in thepresent invention which provides the capability of printing at a farhigher shade density (up to percent), than was heretofore possible.Further advantages of the invention derive from its ability to beinexpensively fabricated.

These and other objects of the invention as well as the features andadvantages thereof will become apparent from the following detailedspecification, when read in connection with the drawings, in which:

FIG. I is a perspective drawing (partially in block form) of a portionof a preferred embodiment of the present invention.

FIG. 2A is an end view of the electrode structure shown in FIG. 1.

FIG. 2B discloses a segment of the recording medium showing the latentcharge pattern for the letter E.

FIGS. 3A through 3B show the various stages of fabrication of a dualelectrode structure.

FIG. 4 is a block diagram of the electrode structure and its associateddrive circuitry.

FIG. 5 is a circuit diagram of one embodiment of the electrode drivecircuitry.

FIG. 6 is a circuit diagram of the driver shown in FIGS. 5 and 7.

FIG. 7 is a circuit diagram of a second embodiment of the electrodedrive circuitry.

ELECTRODE STRUCTURE FIG. 1 is a perspective view of a portion ofapreferred printing system in accordance with the present invention,showing an electrode structure 10 having a section thereof cut awaythereby exposing electrodes l2. For theillustrative embodiment shown,each electrode has a substantially square cross section and terminatesin a working surface I4, which lies within a common surface in closeproximity to a recording medium 20. In one practical embodiment, surface14 is 0.005 inch square and the electrode rows, as well as adjacentelectrodes within a row, are separated by a distance (d) of 0.005 inch.

In the preferred embodiment of the invention illustrated, two rows ofelectrodes 15 and 17 are used. Eachworking surface 14 (except those atthe end of the rows), is aligned with More specifically, a scan line canbe represented as an imaginary line on the medium having the width of anelectrode, as shown in FlG. 28. Given an upper limit on the speed withwhich a row of electrodes can be energized to print a latent image onthe scan line above it, the minimum spacing between electrode rows isdetermined by the paper speed and r the completion of printing of asingle scan line. Such printing must be completed before that scan linemoves above the second row of electrodes.

Rollers l6 and 18 show a means of propelling medium 20 past electrodestructure 10. Various other propelling means can be used all of whichfall within the spirit and scope of the present invention. Medium 20 caninclude a conductive base substrate, such as treated paper havingaffixed thereto a dielectric layer of a prescribed thickness, usuallythinner in dimension than the base material. The dielectric side ofmedium 20 faces roller 16, whereas the conductive side faces roller 18.

As previously mentioned, the generation (printing) of a latent imageoccurs when a high potential is applied across the recording medium at apredetermined place on the medium. Over this predetermined area (thearea above the electrode surface 14), an electrostatic charge transfertakes place and the dielectric retains this charge pattern for asufficient period so that a toner can be applied and fused to the mediumin areas where the charge is present. This toning steps makes the latentimage visible.

With the system shown in FIG. 1, a source 19 provides the high voltagenecessary for printing. This high voltage is coupled to a roller 18 byway of a commutating brush 21, which is adapted to apply the highvoltage to roller 18, and is then applied to the conductive side ofmedium 20. As the medium moves in the direction indicated by arrow 23 ata speed of, for example, inches/second, selected electrodes are pulsedto ground by electrode drive circuitry 24 thereby creating a charge onthe dielectric surface of medium 20 in a particular pattern. A charactergenerator 26 is connected to electrode drive circuitry 24 and determinesthe configuration or printing pattern..

Generator 26 receives suitable electrical waveforms (not shown), e.g.from a computer, representative of pictorial, alphanumeric, or otherinformation to be recorded and converts these waveforms into timed anddistributed electrical pulses which are applied to circuitry 24.Configuration generator 26 may be a typical function generator, such asdisclosed in U.S. Pat. No. 3,289,030 to Lewis et al.

By way of illustration, let it be assumed that printing of a latentimage is to take place in section 21 of medium 20. The portion ofsection 21 that has either passed electrode structure 10, or is aboveit, is shown shaded so as to indicate where the latent image has beenformed. Actually the dielectric surface of medium 20 retains the charge.For the purposes of illustration, however, the side of medium 20 visiblein FlG. 1 is shaded. Directly above electrode structure 10, the latentimage is tooth-shaped, as shown.

At one point in time in the operation of the system, the first electroderow has been energized and the second electrode row 17 has not yet beenenergized. Had only one electrode row been used for printing, therequired minimum spacing between the adjacent electrodes in a row wouldhave yielded a printing density no higher than approximately 60 percent.With the staggered, dual electrode structure shown in FIG. 1, however, aprinting density of up to to 100 percent can be attained. The use ofthis particular structure also allows for increased resolution,particularly when printing curved alphanumeric characters. This improvedresolution is due primarily to the fact that twice as many scans existin the direction of paper motion as a result of the staggered electrodearrangement, than is the case where a single electrode row is used. As aconsequence, better character definition is provided by the presentinvention.

PRINTING EXAMPLE As an aid to a better understanding of the printingsystem of the present invention, reference is directed to FlGS. 2A and28. FIG. 2A shows an end view of the staggered dual electrode structurewhile FIG. 2B shows a segment of medium 20 on which a character has beenprinted. The segment is shown dissected into imaginary cells (elements)30 dimensioned identically to the electrode surface 14 (0.005 inchsquare, for example). The entire character segment consists of a groupof cells arranged in a [6X25 matrix. To provide intercharacter spacing,a 13X l 6 array of cells defines the particular character, an E-shapedcharacter in the example of FIG. 2B.

The electrodes shown in FIG. 2A have been designated a through 11 forelectrode row 15 and a through h for electrode row 17. Like designationsare shown along scan line 1 of FIG. 23. Assuming that the electrodestructure 10 remains stationary and that the segment of medium 20 shownin H0. 28 is about to pass over the electrodes in the direction of thearrow showing paper motion, the following occurs. As determined bycharacter generator 26, for printing the character E, when scan line 1is positioned above first electrode row 15 electrodes a through g areexcited, but electrode 11 is not. The cells designated a through g areelectrostatically charged, but cell h is not. When scan line 2 is abovethe first electrode row 15, the same electrodes are excited (a through3). Simultaneously, scan line 1 is located over an interelectrodesurface 25, which is part of the aforesaid common surface. Electrode row17 has not yet been excited at this time.

At a later time, when scan line 3 has moved directly above the firstelectrode row 15, only electrode a is excited. Simultaneously, scan line1 has moved above the second electrode row 17. Electrodes a through fare now excited but electrodes g and h are not. This procedure repeatsin a manner to permit the printing of latent images in the shaded areasof FIG. 2B. The character generator 26 programs the excitation sequenceof each electrode row. Thus, it is possible to print alphanumericcharacters, special characters, and virtually any other desiredpatterns.

Although only a segment of medium 20 is shown in the drawing, in apractical character-printing system I32 characters may be printed in ahorizontal direction. With a possible 25 scans being used to complete acharacter, 132 characters (a character line) may be printed for every 25scans. The number of electrodes needed to accomplish this is determinedas follows. With 132 character positions and 16 cells per characterposition, there are a total of 2,112 electrodes per scan line (1,056electrodes per electrode row). If it is desired to print 5,000 lines ofcharacters per minute, with the paper moving at approximately 10inches/second (maximum character height of one-eighth inch), it takes l2milliseconds to print a character line (60 see/min. 5,000 characterlines/min) If 25 scan lines per line of characters are used, a scan lineis printed in 0.48 milliseconds (12 milliseconds 25 scan lines).

In order to obtain a print of good quality, a print pulse width ofbetween 40 and 50 microseconds is needed. The pulse width is determinedprimarily by the RC time constant of the electrographic medium. Using aminimum pulse width of 40 microseconds, one could use 12 (0.48milliseconds 40 microseconds) print intervals to print a scan line.Using a pulse width of 50 microseconds, only 9.6 print intervals perscan line are required. If one chooses an intermediate value of llintervals, one can use ll intervals each of 12 character positions. Theprinting pulse for this particular example would be approximately 43.5microseconds.

Thus, in a practical embodiment of the invention, one scan line mayinclude 132 character positions, and 25 scan lines will complete theprinting of the entire line of characters. The 132 character positionsare printed in ll intervals. During each such interval, 12 characterpositions are printed, i.e. l92 (l2 l6) cells are printed (correspondingto 192 electrodes). Each print interval thus takes 43.5 microseconds anda total of 12 milliseconds is needed to print an entire character line.With the dual electrode row operation explained with particularreference to FIG. 2B, the I92 electrodes that are excited at one time toprint the l2 character positions. physically constitute two rows eachhaving 96 electrodes which are staggered as shown in FIG. 2A.

Electrode Fabrication The electrode structure of the present inventioncan be fabricated in various ways. The particular electrode crosssection need not be square in shape. but can instead be rectangular,circular or have various other shapes. FIG. 3A shows a portion of aprinted circuit board 40 having two conductive copper layers 42separated by an insulative layer 44 of glass epoxy or like material.These three layers are affixed together by glueing or other suitablemeans.

One particular way of fabricating the electrode structure is to startwith an etched printed circuit board having copper conductors on bothsides which, in a preferred embodiment, are 0.005 inch in width andspaced 0.005 inch apart (see FIG. 3B). The techniques used in makingthis etched board are similar to those used in manufacturingconventional printed circuits. However, due to the small width of theconductors (electrodes) and the small interconductor spacing certainprocess control is required.

The first step in fabricating the electrode structure is to deposit aphotoresist on both sides of the copper-glass epoxycopper laminate.After suitable cleaning and drying of the board, a negative is alignedwith the coated board (one on each side) and it is exposed toultraviolet light. The board is then developed in a conventionaldeveloper. The final step in obtaining the structure of FIG. 3B is achemical etching step. This is accomplished by immersing the board in awarm 40 percent solution of a mild acid such as ferric chloride. Afterwashing to remove all traces of the acid and drying, the board is readyfor the next fabrication step.

Subsequently the gaps between strips 52 must be filled with epoxy. Epoxysheets 50 are shown abutting against strips 52 in FIG. 3C.

Externally, glass epoxy sheets 46 are placed against sheets 50. Spacers48 determine the extent to which sheets 46 can be pressed against sheets50. When the assembly is heated and pressure is applied, as shown byarrows 54, the epoxy sheets 50 melt and fill the cavities between strips52. After the structure has cooled, a grinding and polishing operationtakes place to obtain the final electrode structure 57 shown in FIG. 3D.An end view of the FIG. 3D structure is shown in FIG. 3E along withrollers 56 and medium 58.

ELECTRODE DRIVE CIRCUITRY As previously mentioned, prior artelectrographic printing systems use a driver circuit for each electrodethat is to be excited. Because of the high voltage levels required forprinting (in the vicinity of 750 volts), sharing of the high-voltagedriver circuits proved unsuccessful heretofore.

FIG. 4 is a block diagram showing electrode drive circuitry 24 andelectrode structure 10. A preferred implementation of circuitry that canbe used as electrode drive circuitry 24 is shown in FIGS. 5, 6 and 7.Electrode drive circuitry 24 has position inputs 70 and data inputs 72.For the practical embodiment of the invention discussed hereinabove, theposition inputs number I 1, corresponding to the 11 printing intervals,while the data inputs number 192, corresponding to the 192 electrodesexcited to print 12 character positions. (Refer to printing exampleabove). Output lines 74 connect from electrode drive circuitry 24 toelectrode structure 10, l I such connections being shown. However, inreality each connection includes l92lines capable to excitation of theircorresponding segments of electrode structure I0.

FIG. 5 disclosed one embodiment of a part of electrode drive circuitry24, including drivers 76, data inputs 72, position inputs 70, outputs 74and a resistor matrix. Resistor pairs 81a, 81b through 89a, 89b connectindividually in series with their common joining node being referred toas nodes 81 through 89 respectively. Nodes 81 through 89 then connectexternally to output line 74. The other terminals of resistors 81a, 84aand 87a respectively connect in common to a a driver 76, while theterminal of resistor trios 82a, a, 88a; 83a, 86a, 89a; 81b, 82b, 83b;84b, 85b, 86b; and 87b, 88b, 8% each connect in common to the otherdrivers 76 of FIG. 5. In this embodiment, all resistors are ofapproximately the same value. The drivers that receive the data lineinputs 72 are continuously switching with each new data scan linepresented. More than one data driver can be and in most cases is, activeat one time. The drivers that receive the position inputs 70 on theother hand, are active, one at a time.

The printing scheme of FIG. 1 used with the resistive matrix of FIG. 5requires that the electrode (roller 18) on the conductive side of themedium be biased at a high voltage of, for example, 700 volts. The pinelectrodes which face the dielectric side of the medium, when switchedto ground, provide the necessary high voltage for printing.

In presently available electrographic printing systems, there isinsufiicient charge established on the dielectric surface of the mediumto attract and hold the toner when the applied voltage across thedielectric is of the order of one-half of the usual 700-volt potential.Thus, the 350-volt difference can be considered to be a threshold valuebelow which successful printing will not occur.

This fact is taken advantage of in the present invention, as shownbelow. The drivers shown in FIGS. 5, 6 and 7 have a binary output of 900volts for nonselect operation and 0 volts for select operation. The datadrivers, therefore, have either 900 or 0 volts at their outputs. Sinceonly one position driver of a total of l 1 drivers is on at any onetime, this driver will have an output of 0 volts while the remainder areat 900 volts. It will be understood that the position inputs 70 for allthe position drivers are sequentially energized by character generator26.

Referring to FIG. 5 in particular, assume that the left position driver76 is selected along with the uppermost data driver 76. The outputs fromthese two drivers would therefore be at ground potential and the voltageat node 81 would be essentially ground. In a practical embodiment, theelectrode on the other side of the medium is biased to 700 volts. Theelectrode associated with node 81 then prints. The nodes 85, 86, 88 and89 are then at 900 volts and no printing occurs. (There is actually areverse 200volt potential difference across the recording medium.) Theremaining nodes, 82, 83, 84, 87 are at one-half of 900 volts or 450volts. The potential difference across the medium in that case is 250volts, (i.e. 700 volts, 450 volts) which is well below the thresholdvoltage of 350 volts. Node 81 is the only one, therefore, that has thecorrect potential applied thereto to facilitate printing.

It will be understood that position inputs 70 are sequentially energizedfor the drivers 76 as a result of the action of character generator 26.As a result, the action described above will occur in sequence, i.e. thenodes will be selected sequentially in groups of threes, i.e. nodes81-82-83; 84-85-8 6; and 87-88-89. Similarly, the outputs 74 will beselected sequentially in accordance with the above sequence and with theselected data input.

FIG. 6 shows a preferred circuit configuration for the driver 76. Theinput at tenninal is a O-volt or +l5-volt signal. The input signal isnormally at ground and goes to the +15- volt level for selection (output128 goes toward ground for selection). A diode 112 has its cathodeconnected to an input terminal I10 and its anode connected in common tothe anode of a diode 116. A resistor 114 connects from the anodes ofdiodes 112 and 116 to a power supply +V,. The parallel combination ofresistor 118 and capacitor 119 connect between the cathode of diode 116and the anode of a diode 120. The cathode of diode I20 connects to thebase of transistor I24 while resistor 122 is coupled from the base oftransistor 124 to power supply V,. Transistors 124 and 126 connect inseries transistor 124 coupled to the emitter of transistor I26 and thecollector of transistor 126 connect via resistor 130 to highpotentialsupply +V,. Output terminal 128 is connected to the collector oftransistor 126. A resistor 132 ties from the base of transistor 126 to ahigh-potential supply +V,, a resistor I34 connects from the base oftransistor 126 to ground and a capacitor 133 connects from the base oftransistor 126 to ground.

In operation, when driver 76 is not selected, input terminal 110 is heldat ground potential and a forward current of approximately 3.5 ma. flowsthrough diode 112 and resistor 114. Little or no current flows in diodes116 or 120 and the slight negative bias on the base of transistor 124determined primarily by resistor 122, maintains transistor 124 turnedoff. Transistor 126, which is rendered capable of conduction by thepositive bias on the base of transistor 126 (resistors 132 and 134 inpart provide the positive bias); is maintained in its off conditionbecause there is no path to ground, i.e. transistor 124 isnonconductive.

When driver 76 is to be selected, the voltage applied to input terminal110 goes to approximately +l5 volts. Diode 112 becomes back biased,while diodes I16 and 120 conduct. Current ilows from source +V,, throughresistor 114, diode 116, resistor-capacitor pair 118, 119, diode 120 andresistor 122 to source V,. Due to the preselected values of resistors114, I18 and 122 (the resistance of resistors 122 is greater than theresistance of resistor 114 plus resistor 118), the base voltage oftransistor 124 becomes positive, thereby turning transistor 124 on. Thisaction is speeded up by bridging resistor 118 and by capacitor 119.

When the input signal goes positive therefore, capacitor 119instantaneously shorts resistor H8 and transistor 124 is rapidlysaturated. This action causes transistor 126 to conduct due to thepositive base voltage established by resistors 132 and 154 and capacitor133. The voltage output at terminal 128 which was at approximately +V(+900 volts, for example) now assumes a value of approximately volts(slightly positive). This voltage is supplied by way of output resistor130.

In FIG. 7 there is disclosed another embodiment of electrode drivecircuitry, corresponding reference numerals having been retained. Asshown, drive circuitry 24 includes drivers 76, data inputs 72, positioninputs 70, sequential outputs 74 and a diode-resistor matrix. Diodepairs 91a, 91b through 99a, 99b connect individually in series withtheir cathodes being connected to nodes 91 through 99, respectively.Nodes 91 through 99 then connect externally to sequential output lines74 and also, respectively to one side of resistors 910 through 99c. Theother terminals of resistors 910 through 99c connect to groundpotential. The anodes of the diodes 91a, 94a and 97a connect in commonto a driver 76 while the anodes of diode trios 92a, 95a, 98a; 93a, 96a,99a; 91b, 92b, 93b; 94b, 95b, 96b; and 97b, 98b, 99b each connect incommon to the other drivers 76 of FIG. 7. The drivers that receive thedata line inputs 72 are continuously switching with each new data scanline presented. More than one data driver can be, and in most cases is,active at one time. The drivers that receive the position inputs 70 onthe other hand, are active one at a time.

Referring to FIG. 7, assume that the left position driver 76 is selectedalong with the uppermost data driver 76. The outputs from these twodrivers (refer to FIG. 6) are therefore at ground potential and thevoltage at node 91 is essentially at ground, (diodes 91a and 91b arereverse biased). With the electrode on the other side of the medium at700 volts, the electrode associated with node 91 prints. The nodes 95,96, 98 and 99 are therefore at 900 volts and no printing occurs. Inpractice, there is actually a reverse ZOO-volt potential differenceacross the medium. The remaining nodes, 92, 93, 94, 97 are at one-halfof 900 volts or 450 volts. The potential difference across the medium isthat case would 250 volts (700-450 volts) which is well below thethreshold voltage of 350 volts. Node 91 is the only one, therefore, thathas the cor rect potential applied thereto to facilitate printing.

The present invention has been described with reference to certainillustrative embodiments. It should be understood, however, thatmodifications may be made in the apparatus described which lie wellwithin the scope of the present invention. For example, a structureusing more than a pair of electrode rows could be used advantageously.If three rows were used, for instance, the individual electrodes in eachrow could be spaced somewhat further apart. Also, the voltage levels andpolarities need not be as set forth in the illustrative example. Apotential difference of 700 volts may be needed for printing. However,the roller could be kept at ground and the pin electrodes may beselectively pulsed to the high voltage, either positive or negative.Further, the common surface of the electrode structure can assumevarious shapes.

From the foregoing it becomes apparent that the apparatus of the presentinvention provides an improved electrographic printing system. Thestaggered multiple row electrode structure provides for improvedresolution and for the possibility of obtaining 100 percent shadedensity. This is particularly advantageous when printing alphanumericcharacters. The electrode drive circuitry also furnishes additionaladvantages in that fewer drivers are needed than in presently availablesystems, with an attendant cost savings. Improved reliability and costsavings is also a feature of the present invention, particularly withreference to the above-illustrated fabrication techniques of theelectrode structure.

Having now described the invention, what is claimed as new and novel andfor which it is desired to secure Letters Patent ls:

I. An electrographic printing system of the kind wherein a recordingmedium is moved along a path to have latent images formed thereon by theapplication of a high potential across the medium and wherein a toner issubsequently applied to the medium to make the latent image visible,said electrographic printing system comprising:

a. an electrode structure adjacent said path including 1. a plurality ofmutually spaced rows of electrodes, successive electrodes within eachrow being spaced from each other, the electrodes of successive rowsbeing positioned in a staggered manner with respect to each other,

2. a single steady-state potential means disposed adjacent the oppositeside of said path and extending substantially for an entire electroderow width for imparting a continuous potential across the medium overthe total medium area covered by said electrode rows, and

b. electrode drive circuitry for selectively energizing each electrodeindividually including 1. a character generator 2. first and secondgroups of high-voltage drivers connected to be energized by saidcharacter generator,

3. a selection matrix including a first plurality of passive elementscoupled to the output of each of said first group of high-voltagedrivers, a second plurality of passive elements coupled to the output ofeach of said second group of high-voltage drivers, each of said elementscoupled to one of said first group of drivers being connected to form acommon node with a separate element coupled to one of said second groupof drivers, an output line connecting each of said nodes to one of saidelectrodes, each of said output lines being selectively adapted to applya high voltage to its corresponding electrode in dependence upon theoutput of said character generator.

2. An electrographic printing system as defined in claim I wherein thespacing between successive rows of electrodes is substantially equal tothe width of each of said electrodes in a direction transverse to saidrows.

3. An electrographic printing system as defined in claim I wherein eachof said passive elements includes a resistor connected between theoutput of a high-voltage driver and said common node.

4. An electrographic printing system as defined in claim 1 wherein eachof said passive elements includes a diode connected between the outputof said high-voltage driver and said common node, said common node beingresistively coupled to a reference potential.

5. An electrographic printing system as defined in claim 1 wherein eachof said high-voltage drivers has an input terminal and an outputterminal and further includes a. a first transistor having base, emitterand collector electrodes, said emitter being tied to ground and saidbase being coupled to said input terminal; and

b. a second transistor having base, emitter and collector electrodes,said last-recited emitter being connected to the collector electrode ofsaid first transistor, said lastrecited base being coupled to areference potential. and said last-recited collector being coupled tosaid output terminal.

6. The printing system of claim 1 wherein said steady-state potentialmeans is a roller employed to feed said medium along the prescribedpath.

7. The printing system of claim 1 wherein each of said electrodesterminates in a common surface and said common surface is convex.

8. The printing system of claim 1 wherein said steady-state potentialmeans is biased at a voltage of less magnitude than said high voltageapplied to said electrodes when energized and greater than the voltageapplied to said electrodes when not energized.

9. An electrographic printing system as defined in claim 1 wherein saidelectrodes terminate in a common surface, each electrode defining aworking surface within said common surface, said working surfaces beingaligned with respective spaces between the electrodes of successive rowsand being dimensioned to substantially fill said spaces.

10. An electrographic printing system as defined in claim 9 wherein eachof said working surfaces has a square shape.

11. In an electrographic printing system of the kind wherein a recordingmedium is moved along a path to have latent images formed thereon by theapplication of a high potential across the medium and wherein a toner issubsequently applied to the medium to make the latent image visible, anelectrode structure adjacent said path including, a plurality of mutually spaced rows of electrodes, successive electrodes within each rowbeing spaced from each other, the electrodes of successive rows beingpositioned in a staggered manner with respect to each other, means forselectively energizing individually, and a simple steady-state potentialmeans disposed adjacent the opposite side of said path and extendingsubstantially for an entire electrode row width for imparting acontinuous potential across the medium over the width of an electroderow, wherein said single steady-state potential means is effective toprovide a continuous potential to the medium for each successive row ofsaid plurality of rows.

12 The apparatus as defined in claim I1, and further including electrodedrive circuitry adapted to selectively energize said electrodes byapplying a high voltage thereto.

13. The printing system of claim 11 wherein said steadystate potentialmeans is a roller employed to feed said medium along a prescribed path.

14. The apparatus as defined in claim 11 wherein said electrodestructure consists of a pair of spaced rows of electrodes, eachelectrode of a row being aligned with the space defined between theelectrodes of the other row and having substantially the same dimensionin a direction along said rows.

15. The apparatus of claim 14 wherein each of said electrodes terminatesin a square working surface lying within a common surface, the spacingbetween said rows and between adjacent electrodes within a row beingsubstantially equal to the side of one said square working surfaces.

16. The apparatus of claim 15 wherein said common surface defines aplane.

17. The apparatus of claim 15 wherein said common surface is convex.

1. A PLURALITY OF MUTUALLY SPACED ROWS OF ELECTRODES, SUCCESSIVEELECTRODES WITHIN EACH ROW BEING SPACED FROM EACH OTHER, THE ELECTRODESOF SUCCESSIVE ROWS BEING POSITIONED IN A STAGGERED MANNER WITH RESPECTTO EACH OTHER,
 1. A CHARACTER GENERATOR
 2. FIRST AND SECOND GROUPS OFHIGH-VOLTAGE DRIVERS CONNECTED TO BE ENERGIZED BY SAID CHARACTERGENERATOR,
 2. A SINGLE STEADY-STATE POTENTIAL MEANS DISPOSED ADJACENTTHE OPPOSITE SIDE OF SAID PATH AND EXTENDING SUBSTANTIALLY FOR AN ENTIREELECTRODE ROW WIDTH FOR IMPARTING A CONTINUOUS POTENTIAL ACROSS THEMEDIUM OVER THE TOTAL MEDIUM AREA COVERED BY SAID ELECTRODE ROWS, AND B.ELECTRODE DRIVE CIRCUITRY FOR SELECTIVELY ENERGIZING EACH ELECTRODEINDIVIDUALLY INCLUDING
 2. a single steady-state potential means disposedadjacent the opposite side of said path and extending substantially foran entire electrode row width for imparting a continuous potentialacross the medium over the total medium area covered by said electroderows, and b. electrode drive circuitry for selectively energizing eachelectrode individually including
 2. first and second groups ofhigh-voltage drivers connected to be energized by said charactergenerator,
 2. An electrographic printing system as defined in claim 1wherein the spacing between successive rows of electrodes issubstantially equal to the width of each of said electrodes in adirection transverse to said rows.
 3. An electrographic printing systemas defined in claim 1 wherein each of said passive elements includes aresistor connected between the output of a high-voltage driver and saidcommon node.
 3. a selection matrix including a first plurality ofpassive elements coupled to the output of each of said first group ofhigh-voltage drivers, a second plurality of passive elements coupled tothe output of each of said second group of high-voltage drivers, each ofsaid elements coupled to one of said first group of drivers beingconnected to form a common node with a separate element coupled to oneof said second group of drivers, an output line connecting each of saidnodes to one of said electrodes, each of said output lines beingselectively adapted to apply a high voltage to its correspondingelectrode in dependence upon the output of said character generator. 3.A SELECTION MATRIX INCLUDING A FIRST PLURALITY OF PASSIVE ELEMENTSCOUPLED TO THE OUTPUT OF EACH OF SAID FIRST GROUP OF HIGH-VOLTAGEDRIVERS, A SECOND PLURALITY OF PASSIVE ELEMENTS COUPLED TO THE OUTPUT OFEACH OF SAID SECOND GROUP OF HIGHVOLTAGE DRIVERS, EACH OF SAID ELEMENTSCOUPLED TO ONE OF SAID FIRST GROUP OF DRIVERS BEING CONNECTED TO FORM ACOMMON NODE WITH A SEPARATE ELEMENT COUPLED TO ONE OF SAID SECOND GROUPOF DRIVERS, AN OUTPUT LINE CONNECTING EACH OF SAID NODES TO ONE OF SAIDELECTRODES, EACH OF SAID OUTPUT LINES BEING SELECTIVELY ADAPTED TO APPLYA HIGH VOLTAGE TO ITS CORRESPONDING ELECTRODE IN DEPENDENCE UPON THEOUTPUT OF SAID CHARACTER GENERATOR.
 4. An electrographic printing systemas defined in claim 1 wherein each of said passive elements includes adiode connected between the output of said high-voltage driver and saidcommon node, said common node being resistively coupled to a referencepotential.
 5. An electrographic printing system as defined in claim 1wherein each of said high-voltage drivers has an input terminal and anoutput terminal and further includes a. a first transistor having base,emitter and collector electrodes, said emitter being tied to ground andsaid base being coupled to said input terminal; and b. a secondtransistor having base, emitter and collector electrodes, saidlast-recited emitter being connecTed to the collector electrode of saidfirst transistor, said last-recited base being coupled to a referencepotential, and said last-recited collector being coupled to said outputterminal.
 6. The printing system of claim 1 wherein said steady-statepotential means is a roller employed to feed said medium along theprescribed path.
 7. The printing system of claim 1 wherein each of saidelectrodes terminates in a common surface and said common surface isconvex.
 8. The printing system of claim 1 wherein said steady-statepotential means is biased at a voltage of less magnitude than said highvoltage applied to said electrodes when energized and greater than thevoltage applied to said electrodes when not energized.
 9. Anelectrographic printing system as defined in claim 1 wherein saidelectrodes terminate in a common surface, each electrode defining aworking surface within said common surface, said working surfaces beingaligned with respective spaces between the electrodes of successive rowsand being dimensioned to substantially fill said spaces.
 10. Anelectrographic printing system as defined in claim 9 wherein each ofsaid working surfaces has a square shape.
 11. In an electrographicprinting system of the kind wherein a recording medium is moved along apath to have latent images formed thereon by the application of a highpotential across the medium and wherein a toner is subsequently appliedto the medium to make the latent image visible, an electrode structureadjacent said path including, a plurality of mutually spaced rows ofelectrodes, successive electrodes within each row being spaced from eachother, the electrodes of successive rows being positioned in a staggeredmanner with respect to each other, means for selectively energizingindividually, and a simple steady-state potential means disposedadjacent the opposite side of said path and extending substantially foran entire electrode row width for imparting a continuous potentialacross the medium over the width of an electrode row, wherein saidsingle steady-state potential means is effective to provide a continuouspotential to the medium for each successive row of said plurality ofrows. 12 The apparatus as defined in claim 11, and further includingelectrode drive circuitry adapted to selectively energize saidelectrodes by applying a high voltage thereto.
 13. The printing systemof claim 11 wherein said steady-state potential means is a rolleremployed to feed said medium along a prescribed path.
 14. The apparatusas defined in claim 11 wherein said electrode structure consists of apair of spaced rows of electrodes, each electrode of a row being alignedwith the space defined between the electrodes of the other row andhaving substantially the same dimension in a direction along said rows.15. The apparatus of claim 14 wherein each of said electrodes terminatesin a square working surface lying within a common surface, the spacingbetween said rows and between adjacent electrodes within a row beingsubstantially equal to the side of one said square working surfaces. 16.The apparatus of claim 15 wherein said common surface defines a plane.17. The apparatus of claim 15 wherein said common surface is convex.